1. Field of the Invention
The present invention relates to a method for producing a toner for use in image forming methods such as an electrophotographic method, a latent electrostatic recording method, a latent electrostatic printing method, a toner jet system recording method, and the like.
2. Description of the Related Art
A toner for developing a latent electrostatic image is used in image forming methods such as an electrophotographic method.
A production method of such a toner is roughly divided into a pulverization method and polymerization method, and the pulverization method is known as a simpler and more popular method.
In the pulverization method, a binder resin for fixing the toner to a transfer material and a colorant are used, and, if necessary, other materials such as a charge control agent, magnetic material, and a release agent are used. These toner materials are mixed and subjected to melting and kneading. Subsequent to cooling down and solidification, the kneaded mixture is finely pulverized by a pulverizer unit and then the resultant is classified by a classifier unit according to a desired particle size distribution. After that, a fluidizer, etc. is added to the resultant, whereby a toner for image formation is produced. Recently, to meet the demands for high-speed printing, high resolution and the like, such a toner including a resin having a low melting point and a release agent, typically a wax, and having a smaller particle size has been dominant in the market.
In a toner production method using such toner materials, fine particle toner (so called substandard toner) whose particle size is less than a predetermined size is generated in the pulverizing and classifying process. The fine particle toner thus generated is collected for reuse.
For achievement of collection and reuse of such a fine particle toner, there are known a method of recycling the substandard toner to a material mixing process (also referred to as mixing process), and a method of introducing the collected substandard toner in a granulating apparatus, followed by pressurizing at a predetermined pressure, granulating, and then mixing with materials in a mixing process, and the like.
For example, the following inventions have been proposed relating to the above methods. Japanese Patent Application Laid-Open (JP-A) No. 05-034976 discloses a method in which the substandard toner is added to a dispersed mixture obtained in a dispersing and mixing process so as to be reused.
JP-A No. 06-186775 discloses a method in which the fine particle toner is uniformly sprinkled onto intermediate products of the toner after a kneading process, dissolved by the heat retained in the intermediate products of the toner, and subjected to cooling and pulverizing, whereby a toner product having a predetermined grain size is regenerated.
JP-A No. 08-069126 discloses a method in which, a kneading process for the fine particle toner is provided separately from a normal toner production process through which a batch-type and continuous-type extruder with single or double-shaft feed screws, which is the same one used in the normal toner production process and, after that, followed by the normal toner production process or the resultant fine particle toner is processed into a coarse powder, followed by coarse pulverizing process.
JP-A No. 10-161343 discloses a method in which the fine particle toner collected and discharged through a classifying process is heated to be melted or softened, the melted or softened substance is extruded by a pressure extruder, and the extruded substance then is mixed into a kneaded substance before cooling after a kneading process.
Further, as a method of granulating the fine particle toner using a granulating apparatus, JP-A No. 06-266157 discloses a toner manufacturing method using a granulating apparatus.
JP-A No. 2006-259017 discloses a method that uses a degasifier for increasing the bulk density of the fine particle toner before a granulation process.
Japanese Patent (JP-B) No. 3435587 discloses a method in which both granulated fine particles and raw fine particles which is not granulated are reused.
However, in the toner recycle technique disclosed in JP-A No. 05-034976, breakage of resin molecule occurs when the fine particle toner is re-kneaded by a kneader. This may reduce the molecular weight of the resin to cause fixing performance to deteriorate or may reduce mechanical strength thereof to cause durability performance to deteriorate.
Further, in the method disclosed in JP-A No. 06-186775, except for the case where the amount of the fine particle toner to be sprinkled is extremely small, the fine particle toner is merely adhered to the surface of pellet-shaped intermediate products of the toner. In this state, a kneaded substance and fine particle toner are not integrated with each other. As a result, the resultant toner easily rubs off. This is caused due to a low heat exchange rate between the kneaded substance and fine particle toner. The fine particle toner other than that having a low softening point is not melted and softened and, therefore, it is difficult to increase the amount of the fine particle toner to be used.
Further, in the method disclosed in JP-A No. 08-069126, when a large amount of fine particle toner having a particle diameter of 5 μm or less exists, so-called “feed-neck” phenomenon that the powder cannot be fed into an extruder well occurs. As a result, the fine particle toner having a particle diameter of 5 μm or less cannot be quantitatively supplied to the extruder.
Further, in the invention disclosed in JP-A No. 10-161343, energy loss occurs since an electric power is required when a pressure extruder is heated using a heater. Further, a substance softened and/or melted by heating remains in a vessel and is not completely discharged from the apparatus, and a maintenance work for removing the residue needs to be carried out every time one process is completed or before activation of the apparatus after the stop thereof.
Especially, in a toner production method in which a toner material including a binder resin, release agent, and pigment are mixed followed by kneading, pulverizing, and classifying, when the substandard fine particle toner generated in the classifying process is directly subjected to the mixing process, the following problems arise.
That is, the toner produced in a state where the feed-neck phenomenon that the substandard fine particle toner does not bite into the screw portion of a kneader well occurs does not reach a sufficient mixing state as compared to the toner in which the recycled fine particle toner is not used, deteriorating dispersibility of the material. Due to the deterioration in the dispersibility of the material and occurrence of the feed-neck phenomenon, the discharge amount from the kneader becomes extremely small, thereby reducing producing capability.
In the method disclosed in JP-A No. 2006-259017 aiming to solve such a problem, there is a limit on the improvement in productivity.
Especially, also in the granulating process disclosed in JP-A No. 06-266157, granulating capability for the fine particle toner containing carbon black having lubricant effect is low. Therefore, in order to maintain production volume, the number of kneader needs to be increased from the current one. Further, also in the method disclosed in JP-A No. 2006-259017, degassing is carried out in the granulating process, and it can easily be estimated that degassing effect in the hopper of the granulating apparatus for the toner containing carbon black having high lubricant effect is reduced. Accordingly, the granulating capability may be extremely deteriorated. If the granulating capability obtained in this method is applied to the apparatus, the size of the apparatus is increased and, accordingly a space for use in producing the toner is increased, resulting in deterioration of industrial practicality. Further, even if the degassing effect is sufficient in the initial stage, it is gradually reduced due to clogging of the degassing part and the like, making it difficult to constantly obtain a predetermined granulated product. Therefore, a periodical maintenance work needs to be carried out for the degassing part, and further, a cumbersome work is required in order to maintain the granulation performance at a constant level, causing deterioration of productivity and increase in cost, which severely limits its industrial applicability.
Further, while production of a small diameter toner or release agent containing toner is required in recent years as described above, a problem of adherence of a fine particle toner or fusion bonding and adherence of a component such as wax to the inside of the pulverizer and classifier arises. Especially, the toner adherence and toner fusion bonding occur notably in the release agent containing toner, extremely deteriorating toner productivity.
Also in the method disclosed in JP-B No. 3435587, the substandard fine particle toner generated in the above production processes contains silica having high lubricant effect. In the case where the fine particle toner containing silica having high lubricant effect or fine particle toner containing carbon black is directly recycled to a material mixing process, so-called feed-neck phenomenon that the mixed material does not bite into an extruder well occurs. This impedes the progress of the kneading process to significantly deteriorate productivity. Even if a delay of the production process does not occur in the initial state of the kneading process, the delay gradually emerges, resulting in a significant reduction in the productivity.
As described above, there has been proposed no solution for preventing occurrence of the feed-neck phenomenon while obtaining a predetermined particle bulk density when the fine particle toner is granulated using the granulating apparatus employed in the methods described above. Especially, the above methods do not mention how a granulating apparatus is configured and what conditions under which the toner production process is carried out in order to achieve a predetermined particle bulk density, to prevent the feed-neck phenomenon, and to basically omit a maintenance work for an apparatus to be employed. In addition, the above methods do not mention a technique that sets the speed ratio of two press rolls which is proposed in the present invention. Further, an invention that can lead to mass production techniques as the present invention does has not yet been disclosed.